Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging (MRI). Particular embodiments relate to MRI of silicone implants.
Discussion of Art
In magnetic resonance imaging (MRI), human or other animal tissue is subjected to a uniform magnetic field, i.e., a polarizing field B0, so that the individual magnetic moments of particle spins in the tissue attempt to align with the polarizing field, but precess about the field in random order at their characteristic Larmor frequency. If the tissue is subjected to an RF magnetic field, i.e., excitation field B1, which defines an x-y plane and varies at a frequency near a Larmor frequency of selected particles, the net aligned moment, or “longitudinal magnetization” of those selected particles, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment. After B1 is terminated, the tipped spins “relax” back into the precession defined by B0, and, as a result, produce RF signals. The RF signals may be received and processed to form an image. In order to form a pixelated image for human interpretation, gradient magnetic fields, Gx, Gy, Gz, are applied to localize the tissue response to B1. In order to distinguish different types of material within a pixelated image, MRI systems sometimes rely on material characteristics known as TR and TE, which are the times required for the tipped spins of a particular material to relax and to “echo” after removal of B1. The phase shift of a particular material's TE from the TE of water, at a given intensity of B0, is known as that material's “chemical shift.”
MRI is increasingly preferred for medical imaging as it avoids exposing patients to radiation. Although useful for many purposes, current MRI techniques encounter difficulty in imaging silicone implants (such as those are used for cosmetic enhancement or repair) separate from surrounding body fat. For example, breast cancer survivors may have silicone implanted for breast reconstruction. Cosmetic surgeons generally find it desirable to follow up on the installation and fixation of such implants. However, MRI presently has difficulty providing images that distinguish the silicone from surrounding fat. This difficulty arises because fat and silicone, at typical diagnostic MRI intensities of 1.5 T-3 T, are believed to have closely similar chemical shifts, as close as 30 phase degrees offset from each other at typical gradient echo spacings used for fat-water separation.
In view of the above, it is desirable to provide apparatus and methods for obtaining images of in vivo silicone separate from fat. Such apparatus and methods might also be helpful toward images of other distinct chemical species even where certain of the chemical species have relatively close chemical phase shifts (i.e., as close as fat and silicone at 1.5 T magnitude of B0).